Method of fracturing subsurface formations



June 6, 1967 J. L. HUITT ETAL METHOD OF FRACTURING SUBSURFACE FORMATIONSFiled Dec. 28, 1964 BRUCE 8. A4: 62 07/ll //V United States Patent3,323,594 METHUD 6F FRAQTURHNG SUBSURFACE FORMATIONS Jimmie L. Huitt,Glenshaw, and Bruce B. McGlothlin, OHara Township, Allegheny County, Pa,assignors to Gulf Research 8; Development Company, Pittsburgh, Pan, acorporation of Delaware Filed Dec. 28, 1964, Ser. No. 421,208 9 Claims.(Cl. 166-4-2) This invention relates to a method of increasing production from fluid-bearing underground formations penetrated by a well, andmore particularly relates to a method of propping fractures inrelatively soft formations subjected to high overburden pressurescausing excessive embedment of propping agent particles in the face ofthe fracture.

Hydraulic fracturing of underground formations has been widely used inrecent years as a method of stimulating production of fluids fromunderground formations penetrated by wells. In the hydraulic fracturingprocess, a fracturing liquid is pumped down the well. to subject aportion of the formation exposed at the borehole of the well to ahydraulic pressure adequate to cause the formation rock to rupture andthereby result in a fracture extending from the well. Additionalfracturing liquid is pumped down the well and into the fracture toextend it for the desired distance from the well. A slurry of proppingagent particles in a carrying liquid is displaced into the fracture todeposit the particles in the fracture and prevent it from closing whenthe pressure on the carrying liquid is released.

It has been found that fractures of maximum flow capacity can beobtained in hard formations by the deposition of the propping agent inthe fracture in a partial monolayer. The particles of propping agent aredeposited in the fracture in a concentration adequate to hold the facesof the fracture apart while leaving substantial space between theparticles for the flow of formation fluids.

If the underground formation is relatively soft, the particles ofpropping agent may become embedded in the face of the fracture andthereby fail to hold the fracture open. Deformable propping agents whichcompress to provide a larger bearing surface against the face of thefracture have been used to reduce embedment of the propping agent in thefarcture; however, many relatively soft formations are subjected to highoverburden pressures and even the deformable propping agents becomecompletely embedded and are not effective in holding the faces of thefracture apart. Moreover, if the concentration of the particles ofdeformable propping agents in the fracture is high, the openingsbetween'the particles are small and the deformation of the proppingagent particles when subjected to the weight of the overburden tends toclose the openings between the particles and thereby further reduce theflow capacity of the fracture.

This invention resides in a method of forming a multilayer pack ofrigid, substantially spherical propping agent particles in a fracture ina relatively soft formation exposed to an overburden pressureapproaching that sufficient to cause substantially complete embedment ofa full monolayer of propping agent particles. In the process through thefracture Without substantial loss to the adjacent formation until theouter boundary of the sealed faces of the fracture is reached and thenflows: into the formation to deposit the propping agent in the fracture.Loss of liquid into the formation beyond the sealed faces of thefracture results in screening out of the propping agent in the vicinityof the outer boundary of the sealed faces. On continued displacement ofthe suspension of propping agent into the fracture, the particles ofpropping agent build up against the screen-out to form. a multilayerpack of propping agent from substantially the outer boundary of thesealed faces of the fracture to the well. Thereafter pressure in thewell is reduced to allow formation fluids to flow through the fractureinto the well through which such fluids flow to the surface.

In the drawings:

FIGURE 1 is a diagrammatic fragmentary vertical sectional view of a wellfrom which a fracture has been made to extend into the surroundingformation;

FIGURE 2 is a diagrammatic fragmentary vertical sectional view of thewell of FIGURE 1 after a portion nearest the well of the faces of thefracture has been sealed; and

FIGURE 3 is a diagrammatic fragmentary sectional view, similar toFIGURES l and 2, of the well during the deposition of a multilayer ofpropping agent in the fracture.

Referring to FIGURE 1, the lower portionof casing 10 of a well isillustrated penetrating a fluid-bearing forrnvation 12 that is to befractured by the method of this invention. Casing it is shown surroundedby a cement sheath 14 positioned by conventional cementing procedures.As illustrated in FIGURE 1, casing 10 is severed at 16 by any suitablemeans such as mechanical milling or a shaped charge to expose a portionof formation 12. Because this invention is of principal utility in thecreation of fractures of high fluid-carrying capacity in softformations, it will ordinarily be desirable to set and cement casingthrough the fluid-bearing formation to support the formation around theborehole: and to create the fracture through a notch cut through thecasing and extending outwardly into the surrounding formation. Thisinvention is not so limited; however, and may be used when the fractureis created through perforations in the casing, or may be used tofracture formations having adequate strength to allow completion with anopen borehole.

A fracture 18 is initiated from the well by pumping a penetrating liquiddown through casing 10 and increasing the pressure until breakdown ofthe formation occurs. Suitable penetrating liquids are water, heavieraqueous liquids such as brines, and hydrocarbon oils. Frequently, aninitial slug of dilute hydrochloric acid is advantageous as apenetrating liquid. A penetrating liquid is used during the extension asWell as the initiation of the fracture to avoid sealing the faces of theouter extremities of the fracture. The penetrating liquid is pumped intothe fracture at a high rate exceeding 30 barrels per minute, and may exceed barrels per minute if the capacity of the pumping equipmentavailable is adequate, to extend the fracture a desired distance such as.50 to feet, preferably approximately 100 feet from the well. Fracturesof larger radial extent can be used but usually are not economic becausethey involve excessive costs for pumping and wellhead equipment. Thecombination of the high pumping rate and a high density liquid iseffective in scouring the fracture faces and carrying solids washed fromthe fracture faces to the outer extremities of the fracture.

The penetrating liquid is followed by a liquid containing a fluid-lossreducing additive adapted to form a seal 20 on the faces of the fractureadjacent the well. The seal 20, which is formed by filtration of thefluid-loss reducing additive from liquid flowing across the faces of thefracture into the formation 12, greatly retards the loss of a subsequentliquid across the faces of the fracture and causes subsequently injectedliquid to flow radially out- Ward through the fracture beyond the outerboundary of the seal. The liquid containing the fiuidloss reducingadditive, hereinafter referred to as the low fluid loss liquid, ispumped into the fracture at substantially the same rate as thepenetrating liquid used to initiate and extend the fracture.

Any of the conventional fluid-loss reducing additives that will form asubstantially impervious but easily removed seal on the faces of thefracture can be used. Examples of such materials are guar gum, karayagum, blown asphalt, and additives of the type described in U.S. PatentNo. 2,779,735. A preferred fluid-loss reducing additive is silica floursuspended in water gelled with guar gum. The concentration of theadditive in the liquid will depend on the pumping rate, formationcharacteristics, and the particular additive used but usually will be inthe range of .003 to .25 pound per gallon.

It is important to this invention that the amount of fluidloss reducingadditive be such that the seal 20 is formed over only a portion of thefaces of the fracture and leaves the faces 22 of fracture 18 at itsouter extremity unplugged. A method for calculating the fracture areaand the volume of liquid required to seal temporarily a given fracturearea is described in The Petroleum Engineer, volume 31, Nos. 4 and 5,April and May of 1959. Another method is described in the bookletentitled Fracplan issued by Halliburton Company in December of 1960. Itis preferred that the seal extend from the well for a distance of atleast 25 feet to provide a propped fracture of substantial radial extentand not over about 75 percent of the radius of the fracture to insurerapid loss of carrying liquid in the unsealed part of the fracture. Forexample, if the fracture 18 extends radially for a distance of 100 feetfrom the well, it is desirable to form the seal 20 on the faces of thefracture from the Well outwardly a distance of only 25 to 75 feet. Aseal of that extent can be formed by 1000 to 5000 gallons of watergelled with guar gum and containing 0.1 pound silica flour per gallon.

The low fluid loss liquid is followed by a penetrating carrying liquiddevoid of fluid-loss reducing additive having particles of a rigid,substantially spherical propping agent suspended in it. In thisinvention it is desirable that the carrying liquid flow easily into theformation beyond the seal 20 and through the multilayer pack to theouter extremities of the fracture after the screen-out commences. Theterm devoid of fluid-loss reducing additive used in describing thecarrying liquid means that the carrying liquid contains only incidentalamounts of such additives such as may be inadvertently picked up by thecarrying liquid. For most effective use in this invention, it is notdesirable to incorporate any fluid-loss reducing additive in thecarrying liquid. The carrying liquid is displaced into fracture 18 at alow rate, less than barrels per minute, to reduce washing of seal fromthe faces of the fracture. Because of seal 20, little of the carryingliquid is lost from the fracture 18 until it flows beyond the outerboundary of the seal; hence, the carrying liquid is effective incarrying the prop ping agent to the outer limits of the seal 20. Loss ofliquid through the unsealed faces 22 of the fracture causes depositionof the propping agent in the fracture in a multilayer pack 24 at theouter boundary of the sealed portion of the fracture. Continueddisplacement of the carrying liquid with propping agent suspended in itresults in build-up of multilayers of propping agent in the fracturefrom the outer boundary of the sealed portion 20 to the well. As thepack 24 builds up toward the Well, carrying liquid from which thepropping agent has been screened by pack 24 filters through the pack andthen into the formation through unsealed faces 22. A sharp increase onthe pressure on the carrying liquid which occurs when the screenoutfills the fracture all of the way to the well indicates completion ofthe filling of the fracture and deposition within the Well bore.Thereafter pressure in the well is reduced to a pressure below theformation pressure, and the resultant flow of formation fluids flushesthe seal from the faces of the fracture and into the well where it isremoved with the production of the formation fluids. If the formationpressure is not adequate to cause the formation fluids to flow to theWellhead, suitable lifting apparatus is used for producing the fluids.

Because of the small spaces between the particles of propping agent inthe multilayer pack, it is essential that the propping agent be rigid,substantially spherical, have a highly uniform size to provide a packhaving a high permeability, and have a strength which will precludecrushing of the particles. The particles of propping agent may have asize spanning 5 numbers or less within the range of 4 to 40 mesh of theU.S. Sieve Series. Suitable propping agent particle sizes are 8 to 12,12 to 20, and 20 to 40 mesh. To provide a pack of maximum flow capacity,it is highly preferred to use a very narrow range of sizes of particlessuch as a range in which percent or more of the particles pass throughone screen in the U.S. Sieve Series and are retained on the next smallerscreen in the series. The particles should have an average roundness andsphericity of at least 0.8. Roundness and sphericity are defined inStratigraphy and Sedimentation by Krumbein and Sloss, pages 78 through83, published by W. H. Freeman Company, 1951 edition. The propping agentis suspended in the carying liquid in a concentration of 1 to 10lbs/gal. for transporting into the fnacture.

Propping agents which are particularly suitable for use in thisinvention are substantially spherical glass beads ob tained by the rapidquenching of molten glass particles from a temperature in excess of 1800F. to a temperature lower than 900 F. in a gas or in a liquid having aviscosity higher than the viscosity of water. The temperature of thequenching medium should not exceed 400 F. The rapid quenching of moltenglass particles results in substantially spherical glass particleshaving a roundness and sphericity higher than 0.8 and characterized byan L/D ratio, referred to as loading strength, exceeding 50,000 p.s.i.when tested between steel plates having a 35 Rockwell C hardness, whereL is the maximum compressive load in pounds that a particle can carryand D is the diameter of the particle in inches. The rapid quenching ofmolten glass globules allows the production of rigid particles havingL/D ratios as high as 250,000 p.s.i. by suitable adjustment of glasscompositions and treating conditions; however, the rapid quenching iseffective in producing spherical glass particles of the desired strengthfrom soda-lime glasses as Well as other glasses such as borosilicate andlead borosilicate glasses. Because this invention is of principalutility in the propping of relatively soft formations, glass particleshaving an L/D ratio higher than 30,000 p.s.i. are satisfactory and canbe used safely to prop fractures in many formations without danger ofcrushing.

A preferred propping agent for use in this invention is a low density,substantially spherical glassy particle. Such particles, having aspecific gravity as low as about 1.3, can be prepared, for example, bythe rapid quenching of slaglike materials. Metallurgical slags, such asblast furnace slag and silica manganese slag, are suitable for thepreparation of the slag spheres. Suitable propping agents have beenprepared from silica manganese containing 39.2 percent silica, 24.8percent alumina, 13 percent calcium oxide, 12.9 percent manganese, 6.6percent magnesium oxide, and 3.5 percent barium oxide. Alumina silicateslags can also be used for the preparation of the high strength, lowdensity particles. The slags are heated to a temperature about 400 abovetheir melting point and quenched in a liquid having a viscosity greaterthan water or in a solid material such as dry carbon flour to atemperature below 900 F. Such slag particles have a specific gravity inthe range of 1.3 to 1.7 as compared with ordinary glass beads which havea specific gravity of approximately 2.6. By suspending a low densityrigid, substantially spherical propping agent in a carrying liquidhaving a high density, preferably a specific gravity higher than 1.2,such as a solution of sodium chloride, calcium chloride, and zincchloride, the tendency of the propping agent to settle from the carryingliquid is reduced and the carrying of the propping agent to the outerboundary of the sealed portion of the fracture is more surelyaccomplished.

The process of this invention is effective in providing fractures ofhigh flow capacity in those formations which because of the strength ofthe formation and the high overburden pressure to which .it is subjectedcannot be efl'ectively propped with a monolayer of propping agent. Therigidity of the propping agent particles used as well as their highroundness and uniform size provides a multilayer pack having a highpermeability which is not decreased substantially by deformation of theparticles when the pack is subjected to the compressive load of theoverburden. The formation of a substantially impermeable but temporaryseal on only the portion of the faces of the fracture nearest the wellinsures flow of the carrying fluid outwardly for a substantial distancein the fracture before it is lost to the formation, and thereby causesthe initial screen-out to occur at a substantial distance from the welland build the multilayer pack from the outer boundary of the sealedportion to the borehole wall of the well.

We claim:

1. A method for increasing the productivity of a subterranean formationpenetrated by a well comprising pumping a penetrating liquid down thewell to the formation and increasing the pressure on the penetratingliquid to rupture the formation rock and thereby create a fractureextending from the well, pumping liquid containing a fluid-loss reducingadditive in an amount adapted to form a seal on the faces of thefracture outwardly from the well only a portion of the radial extent ofthe fracture, and thereafter pumping into the well and displacing intothe fracture a second penetrating liquid having suspended thereinsubstantially spherical particles of a rigid propping agent, said secondpenetrating liquid having a concentration of particles of propping agentsuspended therein and being pumped at a rate such that screen-out of theparticles occurs in the vicinity of the outer boundary of the sealformed on the faces of the fracture, and continuing the pumping of thesecond penetrating liquid having propping agent suspended therein tofill the fracture from the screen-out to the borehole with a multilayerpack of propping agent particles.

2. A method of increasing the productivity of a subterranean formationpenetrated by a well comprising pumping a penetrating liquid down theWell and into the formation, increasing the pressure on the penetratingliquid to rupture the formation rock and create a fracture extendingoutwardly into the formation from the well, displacing down the well andinto the fracture a low fluid loss liquid containing a fluid-lossreducing additive in an amount adapted to seal the faces of the fractureoutwardly from the well for a distance of at least 25 feet but not over75 percent of the distance from the well to the outer boundary of thefracture, pumping a carrying liquid devoid of sealing material andhaving suspended therein substantially spherical particles of a rigidpropping agent, the rate of pumping said carrying liquid being less thanabout barrels per minute to avoid removing the seal from the faces ofthe fracture, and continuing pumping the carrying liquid containing thepropping agent to form a multilayer pack of propping agent in thefracture adjacent the well.

3. A method of increasing the productivity of a subterranean formationpenetrated by a well comprising pumping a penetrating liquid down thewell into contact with the formation and increasing the pressure thereonto create a fracture extending 50 to 125 feet from the well, thereafterpumping down the well and into the fracture a low fluid loss liquidcontaining a fluid loss reducing additive in an amount to seal the facesof the fracture from the well outward for a distance from a minimum of25 feet to a maximum of 75 percent of the radius of the fracture,displacing down the well and into the fracture at a rate adapted tocause screen out of the propping agent in the fracture a carrying liquidsubstantially devoid of fluid loss additive having suspended thereinglass spheres having a particle size spanning not more than 5 screens inthe range of 4 to 40 mesh in the U.S. Sieve Series, an average roundnessand sphericity of at least 0.8, and a loading strength of at least30,000 p.s.i., and continuing the displacement of the carrying liquiddown the well to build a multilayer pack of propping agent' in thefracture from the outer region of the sealed faces of the fracture tothe well.

4. A method of increasing the productive capacity of a subterraneanformation penetrated by the borehole of a well comprising pumping apenetrating liquid down the well and into contact with the subterraneanformation, increasing the pressure on the penetrating liquid to rupturethe formation rock, continuing pumping the penetrating liquid at a rateof at least 30 barrels per minute to extend the fracture for a distanceof at least 50 feet from the well, pumping a low fluid loss liquid downthe well and into the fracture at a rate of at least 30 barrels perminute, said low fluid loss liquid containing a fluid loss reducingadditive in an amount adapted to seal the faces of the fracture adjacentthe well for a distance from a minimum of 25 feet to a maximum of 75percent of the radius of the fracture, thereafter pumping down the welland into the fracture at a rate less than 10 barrels per minute acarrying liquid having suspended therein glass beads to build amultilayer pack of the glass beads from the outer region of the sealedfaces of the fracture to the well, said glass beads having a. loadingstrength of at least 30,000 p.s.i., an average roundness and sphericityof at least 0.8, and a particle size such that at least percent of theparticles pass through a screen in the range of 4 to 35 mesh in the U.S.Sieve Series and are retained on. the next smaller screen in the series.

5. A method of increasing the productivity of a sub terranean formationpenetrated by a well comprising pumping a penetrating liquid down thewell and into cont-act with the subterranean formation, increasing thepressure on the penetrating liquid to rupture the formation rock andcreate a fracture extending therefrom, pumping down the well and intothe fracture a low fluid loss liquid having a fluid loss reducingadditive incorporated therein, the amount of low fluid loss liquid andfluid loss reducing additive incorporated therein being adapted to sealthe faces of the fracture adjacent the well for a distance of at least25 feet from the well and not exceeding 75 percent of the radius of thefracture, pumping down the well and into the fracture at a rate notexceeding 10 barrels per minute a suspension of glass beads in a brinedevoid of fluid loss reducing additive whereby brine is lost from thefracture adjacent the outer boundary of the sealed portion of thefracture to cause a screen-out of glass beads in the fracture, andcontinuing the pumping of the brine having glass beads suspended thereindown the well and into the fracture whereby the brine filters throughthe screen-out glass particles and builds a multilayer of glassparticles in the fracture to the well bore, said glass beads having aloading strength of at least 50,000 p.s.i. and a particle size spanningnot more than 5 screens in the U.S. Sieve Series between 4 and 40 mesh,and said brine having a specific gravity of at least 1.2.

6. A method as set forth in claim 1 in which the propping agent consistsof glassy particles having a loading strength of at least 30,000 p.s.i.,a specific gravity of 1.3 to 1.7, and a particle size spanning not morethan 5 screens of the U.S. Sieve Series in the range of 4 to 40 mesh.

7. A method as set forth in claim 1 in which the well has casing setthrough the formation and a continuous, substantially horizontal notchis cut through the casing and into the surrounding formation prior topumping the penetrating liquid down the well.

8. A method as set forth in claim 2 in which the rigid propping agent iscomposed of glass beads having a loading strength of at least 50,000p.s.i. and having been prepared by quenching molten glass globules froma temperature of 1800 F. to a temperature below 900 F. in a quenchingmedium selected from the group consisting of liquids having a viscositygreater than the viscosity of Water and gases.

References Cited UNITED STATES PATENTS 3,239,006 3/1966 Fast 166-423,242,032 3/1966 Schott l6642 3,245,866 4/1966 Schott 166--42 CHARLES E.OCONNELL, Primary Examiner.

N. C. BYERS, Assistant Examiner.

5. A METHOD OF INCREASING THE PRODUCTIVITY OF A SUBTERRANEAN FORMATIONPENETRATED BY A WELL COMPRISING PUMPING A PENETRATING LIQUID DOWN THEWELL AND INTO CONTACT WITH THE SUBTERRANEAN FORMATION, INCREASING THEPRESSURE ON THE PENETRATING LIQUID TO RUPTURE THE FORMATION ROCK ANDCREATE A FRACTURE EXTENDING THEREFORM, PUMPING DOWN THE WELL AND INTOTHE FRACTURE A LOW FLUID LOSS LIQUID HAVING A FLUID LOSS REDUCINGADDITIVE INCORPORATED THEREIN, THE AMOUNT OF LOW FLUID LOSS LIQUID ANDFLUID LOSS REDUCING ADDITIVE INCORPORATED THEREIN BEING ADAPTED TO SEALTHE FACES OF THE FRACTURE ADJACENT THE WELL FOR A DISTANCE OF AT LEAST25 FEET FROM THE WELL AND NOT EXCEEDING 75 PERCENT OF THE RADIUS OF THEFRACTURE, PUMPING DOWN THE WELL AND INTO THE FRACTURE AT A RATE NOTEXCEEDING 10 BARRELS PER MINUTE A SUSPENSION OF GLASS BEADS IN A BRINEDEVOID THE FLIUD LOSS REDUCING ADDITIVE WHEREBY BRINE IS LOST FROM THEFRACTURE ADJACENT THE OUTER BOUNDARY OF THE SEALED PORTION OF THEFRACTURE TO CAUSE A SCREEN-OUT OF GLASS BEADS IN THE FRACTURE, ANDCONTINUING THE PUMPING OF THE BRINE HAVING GLASS BEADS SUSPENDED THEREINDOWN THE WELL AND INTO THE FRACTURE WHEREBY THE BRINE FILTERS THROUGHTHE SCREEN-OUT GLASS PARTICLES AND BUILDS A MULTILAYER OF GLASSPARTICLES IN THE FRACTURE TO THE WELL BORE, SAID GLASS BEADS HAVING ALOADING STRENGTH OF AT LEAST 50,000 P.S.I. AND A PARTICLE SIZE SPANNINGNOT MORE THAN 5 SCREENS IN THE U.S. SIEVE SERIES BETWEEN 4 AND 40 MESH,AND SAID BRINE HAVING A SPECIFIC GRAVITY OF AT LEAST 1.2.